xbps_file_hash: reduce memory footprint and process file in small chunks.

With xbps-rindex(8) -c/-r being multithreaded, the memory usage was
insanely high in the build server, with peaks of almost 4GB (this depends
on the file size being processed and number of threads).

Process the file in small chunks instead to compute the SHA256 hash.

lib/util_hash.c

@@ -61,17 +61,16 @@ char *
 xbps_file_hash(const char *file)
 {
 	struct stat st;
+	SHA256_CTX ctx;
 	char hash[SHA256_DIGEST_LENGTH * 2 + 1];
-	unsigned char *buf = NULL, digest[SHA256_DIGEST_LENGTH];
+	unsigned char digest[SHA256_DIGEST_LENGTH];
+	ssize_t ret;
+	unsigned char buf[256];
 	int fd;
 
-	assert(file != NULL);
-
-	if ((fd = open(file, O_RDONLY|O_CLOEXEC)) == -1) {
-		free(buf);
+	if ((fd = open(file, O_RDONLY)) == -1)
 		return NULL;
-	}
-	memset(&st, 0, sizeof(st));
+
 	if (fstat(fd, &st) == -1) {
 		(void)close(fd);
 		return NULL;
@@ -81,20 +80,18 @@ xbps_file_hash(const char *file)
 		return NULL;
 	}
 
-	buf = malloc(st.st_size);
-	assert(buf);
+	SHA256_Init(&ctx);
+	while ((ret = read(fd, buf, sizeof(buf))) > 0)
+		SHA256_Update(&ctx, buf, ret);
 
-	if (read(fd, buf, st.st_size) != st.st_size) {
-	       free(buf);
-	       (void)close(fd);
-	       return NULL;
-	}
-	(void)close(fd);
-	if (SHA256(buf, st.st_size, digest) == NULL) {
-		free(buf);
+	if (ret == -1) {
+		/* read error */
+		(void)close(fd);
 		return NULL;
 	}
-	free(buf);
+
+	SHA256_Final(digest, &ctx);
+	(void)close(fd);
 	digest2string(digest, hash, SHA256_DIGEST_LENGTH);
 
 	return strdup(hash);